Modelling and experimental analysis of the effect of solute iron in thermally grown Zircaloy-4 oxides


Simulations based on density functional theory (DFT) were used to investigate the behaviour of substitutional iron in both tetragonal and monoclinic ${\text{ZrO}\_2}$. Brouwer diagrams of predicted defect concentrations, as a function of oxygen partial pressure, suggest that iron behaves as a p-type dopant in monoclinic ${\text{ZrO}\_2}$ while it binds strongly to oxygen vacancies in tetragonal ${\text{ZrO}\_2}$. Analysis of defect relaxation volumes suggest that these results should hold true in thermally grown oxides on zirconium, which is under compressive stresses. X-ray absorption near edge structure (XANES) measurements, performed to determine the oxidation state of iron in Zircaloy-4 oxide samples, revealed that 3+ is the favourable oxidation state but with between a third and half of the iron, still in the metallic ${{\text{Fe}}^0}$ state. The DFT calculations on bulk zirconia agree with the preferred oxidation state of iron if it is a substitutional species but do not predict the presence of metallic iron in the oxide. The implications of these results with respect to the corrosion and hydrogen pick-up of zirconium cladding are discussed.

Journal of Nuclear Materials
Mark Wenman
Mark Wenman
Senior Lecturer in the Department of Materials